add links to docs

fixes
apache#2149 (comment)

Author: lovasoa

src/ast/mod.rs

@@ -1321,8 +1321,16 @@ impl fmt::Display for LambdaFunction {
 #[cfg_attr(feature = "visitor", derive(Visit, VisitMut))]
 pub enum LambdaSyntax {
     /// Arrow syntax: `param -> expr` or `(param1, param2) -> expr`
+    ///
+    /// https://docs.databricks.com/aws/en/sql/language-manual/sql-ref-lambda-functions
+    ///
+    /// Supported, but deprecated in DuckDB:
+    /// https://duckdb.org/docs/stable/sql/functions/lambda
     Arrow,
     /// Lambda keyword syntax: `lambda param : expr` or `lambda param1, param2 : expr`
+    ///
+    /// Recommended in DuckDB:
+    /// https://duckdb.org/docs/stable/sql/functions/lambda
     LambdaKeyword,
 }